1. Field of the Invention
The invention relates to a sliding door with a magnetic drive system including a displacement measuring system. The magnetic drive system has a linear drive unit with at least one row of magnets. The term “row of magnets” comprises oblong individual magnets as well. The row of magnets can be stationary or non-stationary. Preferably, the magnetic drive system is formed as a magnetic carrying and drive system.
2. Description of the Related Art
A sliding door guide is known from DE 40 16 948 A1, wherein, under normal load, magnets, interacting with one another, effect a contact-free floating guidance of a door leaf or the like, which leaf is maintained in a sliding guide, in addition to the stationary disposed magnets of the sliding guide, a stator of a linear motor being provided, the rotor thereof being disposed at the sliding door. On account of the selected V-shaped disposition of the permanent magnets of the disclosed permanently excited magnetic carrying device, a laterally stable guiding path can not be realized, hence a relatively complicated disposition and configuration of stator and rotor are required.
A combined support and drive system for an automatically operated door is known from WO 00/50719 A1, wherein a permanently excited magnetic carrying system is symmetrically designed and has stationary and non-stationary rows of magnets, which are respectively disposed in one plane, the carrying system being in an unstable equilibrium, and wherein the carrying system has symmetrically disposed lateral guiding elements, which may have roller-shaped supports. The thus achieved laterally stable guiding path results in a simple configuration and disposition of stator and rotor of a linear motor accommodated in a common housing, namely the option of being able to arbitrarily dispose the stator and the rotor of the linear motor in relation to the carrying system and of experiencing no limitations by the carrying system as to the shape of stator and rotor.
These two support systems have in common that they function according to the principle of repulsive forces, which principle of action allows for a stable poise without requiring an expensive electrical control device. However, the drawback therein is that both, at least one stationary and at least one non-stationary row of magnets need to be provided, i.e. magnets need to be disposed along the whole path of the sliding guide or of the bearing of the automatically operated door and at the carrying slide for the door, which slide is movable along this guide, thus making the production of such system very costly, which on the other hand, is distinguished by an extremely soft-running and silent operation and is almost wear-free and maintenance free, as the mechanical friction necessary for carrying the door has been obviated.
Another electromagnetic drive system for magnetic floating and carrying systems is known from DE 196 18 518 C1, wherein a stable floating and carrying state is achieved through an appropriate disposition of a permanent magnet and ferromagnetic material. For this purpose, the permanent magnet brings the ferromagnetic material in a state of partial magnetic saturation. Electromagnets are disposed such that the permanent magnets are moved exclusively by changing the saturation in the carrying rail, and the coil cores are included in the permanent magnetic partial saturation, which results in the floating and carrying state.
WO 94/13055 further shows a stator drive for an electric linear drive and a door, which is equipped with such a stator and suspended by means of magnets at the door lintel of a frame. For this purpose, several magnets or groups of magnets are disposed at the door panel, their magnetic field strength being so important that an attractive force to a guiding plate, disposed at the underside of the door lintel is achieved, whereby this attractive force is sufficient to lift the weight of the door.
The two systems described in these publications have in common that the magnets are prevented from sticking to the ferromagnetic material by means of rollers, that is an air gap between the magnets and the ferromagnetic material is adjusted by means of rollers. In the chosen dispositions, these rollers have to absorb important forces, as the magnetic field strength can not be chosen such that just the respective magnetically suspended door is maintained, but, on account of safety regulations, a predetermined additional portative force needs to be provided to avoid an unintentional drop of the door. Therefore, the rollers need to be designed similarly to purely roller-supported sliding doors, with the result of a mechanical friction occurring when adjusting the air gap. This friction neutralizes the extreme soft-running and silent operation of the support, working according to the principle of repulsive forces, and leads to wear and maintenance. In addition during manufacturing already, the magnetic attractive force needs to be adjusted precisely to the respective load to be carried, therefore these systems are not suitable for the practical application or they are too expensive.
Furthermore, these publications certainly state the use of a linear drive coupled to or integral with a magnetic carrying device; however, the configuration of such a linear drive or the activation thereof are not described.
For operating a linear motor for a sliding door drive, a measuring system for detecting the current position of the door leaf respectively of the rotor is required for several tasks:                1. The rotor position is detected in order to be able to vary the phase voltage depending on the rotor position for commutating the stator such that a continuous motor thrust is generated. Preferably the voltage has a sinusoidal modulation.        2. The travel path is measured for regulating the door speed.        3. The door speed is derived from the position signal for regulating the door speed and for detecting obstacles.        4. The detection of final positions and a measurement of the possible travel path are realized in a reference run.        
With linear motors various analogous and incremental displacement measurement processes are known, which generally are designed as systems independent of the linear motor, such that their measurement results need to be converted to the motor raster for the commutation. Furthermore, a relation between the electrical phase position and the measured rotor position needs to be determined by means of a rotor position search.
Furthermore, it is known that the row of permanent magnets of the rotor can be scanned by means of a Hall sensor or another magnetically sensitive electrical sensor. A system suitable for this purpose is shown in FIG. 20a, in which, seen in the direction of travelling x, in the centre of individual coils 2 of the stator, a Hall sensor 16, serving as a position sensor, is disposed, which emits a signal S1 generated by displacing the individual magnets of the row of magnets 1, having a pole distance R, along the Hall sensor 16. This signal is shown in FIG. 21.
The advantages of such a displacement measuring system in comparison to systems, which are independent of the linear motor, are the following:                1. Cost advantage, because the magnetic scale is already provided and Hall sensors are relatively inexpensive.        2. The rotor position search can be forgone, because a fixed relationship between the measurement signal of the rotor position and the coil positions of the stator of the linear motor is given on account of the mounting position of the Hall sensor. Furthermore, loosing the position relation, which is possible with external systems, is excluded on account of the fixed geometrical allocation.        
The disadvantages, when using a Hall sensor for the displacement detection may be the following:                1. Inherent to design, the length of the row of magnets on the rotor can be shorter than the travel path of the door such that the rotor moves out of the measuring range, as shown in FIG. 22a.         2. On account of geometrical tolerances between the permanent magnets, differing material properties and quality, the maximal field strengths of the individual magnets of a row of magnets have noticeable discrepancies, such that the evaluation is more difficult and the result of the measurement inaccurate. The output signal S1 of the Hall sensor 16 shown in FIG. 21, shows e.g. an amplitude difference D between the third and fourth maximum, which is caused by said reasons.        3. The course of the measured values depending on the rotor position depends on the disposition of the magnets, the choice of sensors and the sensor position. Generally, the output signal of the position sensor is similar to a sinus function, as can be seen clearly seen in FIG. 21, showing the output signal S1 of the Hall sensor 16.        4. Exclusively the steep portions of the sinusoidal result function of the Hall sensor 16 can be evaluated analogously with sufficient precision. In the areas of little slope of the maxima and minima of the functional course, an analogous function evaluation is not possible, as can be clearly seen in FIG. 21 showing the output signal S1 of the Hall sensor 16.        5. With the motor being switched on, the fields of the individual magnets of the row of magnets 1 are superimposed by the fields of the drive coils, such that there is interference in the magnetic fields of the individual magnets of the row of magnets 1 and the result of the measurement is corrupted.        